The present invention is generally related to radiotelephones, and more particularly to an improved speakerphone for cellular telephones.
When a cellular radiotelephone is operating in the full duplex mode, speakerphone audio signals from the speaker audio may be added to the microphone audio within the immediate acoustic environment, in the case of an automobile this would be in the passenger compartment. This new signal can then be propagated through the cellular radiotelephone transmitter, the cellular base station and land line telephone path, back into the receiver side of the cellular radiotelephone and finally back to the speaker audio output to produce an irritating feedback howl. This is due to the gain being added to the acoustical feedback loop by the different parts of the loop, i.e., the cellular telephone, the cellular base station, and the land line telephone system. The tendency to do this is strongly dependent on the land line telephone system to which the the cellular base station interfaces. Many countries regulate the amount of speaker to microphone coupling they will allow because high sidetone levels in their land line telephone systems result in the production of this irritating feedback howl, if the acoustical coupling is not minimized to preclude it. For the foregoing reasons, there is a need for a cellular telephone with a speakerphone that will allow the cellular radiotelephone system to function with land line telephone systems, which are characterized by high sidetone levels that have a propensity to introduce undesirable feedback howl when speakerphones are used.